| Secernosaurus | |
|---|---|
 | |
| Life restoration | |
Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Reptilia |
| Clade: | Dinosauria |
| Clade: | †Ornithischia |
| Clade: | †Ornithopoda |
| Family: | †Hadrosauridae |
| Subfamily: | †Saurolophinae |
| Clade: | †Austrokritosauria |
| Genus: | †Secernosaurus Brett-Surman, 1979 |
| Type species | |
| †Secernosaurus koerneri Brett-Surman, 1979 | |
Secernosaurus (meaning "severed lizard") is agenus ofherbivoroushadrosauriddinosaur that lived during the LateCretaceous of what is nowArgentina. This genus and its close relatives lived in South America, unlike most hadrosaurids, which lived in theLaurasian continents of Eurasia and North America. It has been suggested that the ancestors ofSecernosaurus crossed into South America when a land bridge temporarily formed between North and South America during the Late Cretaceous and allowedbiotic interchange between the two continents.
The holotype ofSecernosaurus koeneri was collected in 1923 as part of an expedition by theField Museum led by J. B. Abbott. However, the specimen was not studied until the 1970s.[1] In 1979, Brett-Surman namedSecernosaurus. Though hadrosaurid specimens from South America had been described before,Secernosaurus koeneri was the first species of South American hadrosaurid to be formally named. The genus nameSecernosaurus means "separated reptile"[2] and comes from the Latin verbsēcernō, meaning to sever or divide, in reference to its geographic location separated from Laurasian hadrosaurs. The species nameS. koerneri honors Harold E. Koerner.[3] The holotype ofSecernosaurus koerneri isFMNH P13423,[1] a partial skeleton from theLago Colhué Huapi Formation of Chubut province, Argentina.[4]
In 2010, Albert Prieto-Marquez and Guillermo Salinas argued thatKritosaurus australis was synonymous withSecernosaurus koeneri.[1] However, in 2015, Rodolfo Coria noted differences between the two, and suggested their taxonomy needed reevaluation.[5] In 2022, the two species were recognized as separate and the genusHuallasaurus was established forKritosaurus australis.[6]
Secernosaurus may have been small for a hadrosaurid. The type specimen pertains to an individual approximately 4–5 metres (13–16 ft) long, which was initially suggested to be a subadult, but has been suggested that it may have been more mature than previously thought.[5] The neuroanatomy ofS. koerneri was very similar to that of hadrosaurids from theNorthern Hemisphere.[7]
Phylogenetic analyses have foundSecernosaurus to be a member of the hadrosauridtribeKritosaurini within the subfamilySaurolophinae. North American animals such asKritosaurus andGryposaurus are also part of thisclade.[1] Rozadillaet al. (2022) recovered all South American saurolophines to group together within a single clade consisting ofSecernosaurus,Huallasaurus,Kelumapusaura, andBonapartesaurus.[6] In the 2023description of the South AmericanhadrosauroidGonkoken, Alarcón-Muñozet al. named theAustrokritosauria, a clade closely related to kritosaurins, consisting of all the South American saurolophines. The results of their phylogenetic analyses of Saurolophinae are displayed in thecladogram below:[8]
| Saurolophinae |
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The geologic layers of theLago Colhué Huapi Formation, whereScernosaurus hails from, have proved difficult to interpret historically, their assignment shifting around between several differentgeologic formations before finally being settled as its own unit of theChubut Group, dating to theMaastrichtian. Additionally,Secernosaurus specifically had a very uncertain geologic provenance. As a result, the ecosystemSecernosaurus would have lived in was not well understood historically. A 2016 paper by Casal A. Gabriel and colleagues studied the climatic conditions of the region more in depth, finding evidence of climate change across geologic time. Fluvial systems and evidence of a floodplain environment were recognized, but the geologically lower parts of the formation also showed evidence of semi-arid conditions, largegypsum deposits, and desiccation cracks, indicating intense aridization of the region compared to the very humid climate of theBajo Barreal Formation's ecosystem that preceded Lago Colhué Huapi Formation. However,palynological data indicate that during the upper deposits of the formation at the very end of theCretaceous and into theDanian age of thePaleocene, the climate became milder once again and returned to a balanced wet and dry season. It is from this uppermost part of the formation thatSecernosaurus is from. More recent discoveries of hadrosaur remains from other localities, which could belong toSecernosaurus might extend the range within the formation hadrosaurs are found in. Currently, however, they have not been researched in depth.[9]
Secernosaurus is not the only dinosaur known from Lago Colhué Huapi. Remains of thesauropodsElaltitan lilloi,Argyrosaurus superbus andAeolosaurus colhuehuapensis, as well as unidentified sauropod remains found in 2010, have been discovered in the formation. Very fragmentary remains ofdromaeosaurid andmegaraptoran theropods such asJoaquinraptor[10] have also been discovered, as is expected from other formations from a similar time and place. The enigmaticornithischian dinosaurNotoceratops, based on a lost fragmentary specimen originally considered to belong to aceratopsian but now debated between that identity and that of a hadrosaur. Finally, scant fossils ofchelonians,crocodyliformes, anddipnoid fish have also been found.[9]
Secernosaurus lived in what is now Patagonia during theMaastrichtian age of the Cretaceous period.[4] It was one of the few hadrosaurs to live in South America. Their presence in South America is likely to represent a dispersal event from North America during theCampanian, when the proto-Antilles may have formed an island chain that allowed land vertebrates to cross between the two continents.[1] The arrival of hadrosaurids in South America may have caused the decline of the native ornithopods, theelasmarians.[4]
